Backend infers play direction; UI previews words and gates submit on legality

A single tile that only extended a word perpendicular to the client-declared
direction was rejected: the UI always sent dir=H for one-tile plays (the
dirOverride/Controls toggle was orphaned in the Stage 7 game rework), so placing
"А" above "БАК" to form "АБАК" failed the solver's main-word-length check even
though the word is in the dictionary.

Make the backend infer a play's orientation from the placed tiles and the board
(internal/engine.resolveDirection): two or more tiles by the line they share, a
lone tile by the axis it abuts (longer word wins, horizontal on a tie). Direction
becomes an output, not an input: drop dir from the SubmitPlay/Eval wire requests
and add it to EvalResult. Journal replay keeps trusting the stored "H"/"V"
(SubmitPlayDir) so a rebuilt game matches the one committed.

UI: stop computing/sending direction; the preview now shows the words a move
forms with its total score (game.previewWords); the make-move control is disabled
until the play is confirmed legal; the "your turn" label hides while tiles are
pending. Delete the orphaned Controls.svelte.

Regenerate the FlatBuffers bindings (Go + TS) and update the gateway transcode
and the loadtest edge client to the new contract. Bake the decision into
ARCHITECTURE.md (§5/§9.1), FUNCTIONAL.md (+ _ru) and the backend README.

pkg/fbs/scrabblefb/EvalRequest.go

@@ -49,16 +49,8 @@ func (rcv *EvalRequest) GameId() []byte {
 	return nil
 }
 
-func (rcv *EvalRequest) Dir() []byte {
-	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
-	if o != 0 {
-		return rcv._tab.ByteVector(o + rcv._tab.Pos)
-	}
-	return nil
-}
-
 func (rcv *EvalRequest) Tiles(obj *PlayTile, j int) bool {
-	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
 	if o != 0 {
 		x := rcv._tab.Vector(o)
 		x += flatbuffers.UOffsetT(j) * 4
@@ -70,7 +62,7 @@ func (rcv *EvalRequest) Tiles(obj *PlayTile, j int) bool {
 }
 
 func (rcv *EvalRequest) TilesLength() int {
-	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
 	if o != 0 {
 		return rcv._tab.VectorLen(o)
 	}
@@ -78,16 +70,13 @@ func (rcv *EvalRequest) TilesLength() int {
 }
 
 func EvalRequestStart(builder *flatbuffers.Builder) {
-	builder.StartObject(3)
+	builder.StartObject(2)
 }
 func EvalRequestAddGameId(builder *flatbuffers.Builder, gameId flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(gameId), 0)
 }
-func EvalRequestAddDir(builder *flatbuffers.Builder, dir flatbuffers.UOffsetT) {
-	builder.PrependUOffsetTSlot(1, flatbuffers.UOffsetT(dir), 0)
-}
 func EvalRequestAddTiles(builder *flatbuffers.Builder, tiles flatbuffers.UOffsetT) {
-	builder.PrependUOffsetTSlot(2, flatbuffers.UOffsetT(tiles), 0)
+	builder.PrependUOffsetTSlot(1, flatbuffers.UOffsetT(tiles), 0)
 }
 func EvalRequestStartTilesVector(builder *flatbuffers.Builder, numElems int) flatbuffers.UOffsetT {
 	return builder.StartVector(4, numElems, 4)